Chapter 7: Catalytic Hydrocracking - PowerPoint PPT Presentation

Chapter 7: Catalytic Hydrocracking. The interest in the use of hydrocracking has been caused by several factors, including The demand for petroleum products has shifted to high ratios of gasoline and jet fuel compared with the usages of diesel fuel and home heating oils,

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The cracking reaction is endothermic and the hydrogenation reaction is exothermic.

The overall reaction provides an excess of heat because the amount of heat released by the exothermic hydrogenation reactions is much greater than the amount of heat consumed by the endothermic cracking reactions

This surplus of heat causes the reactor temperature to increase and accelerate the reaction rate. This is controlled by injecting cold hydrogen as quench into the reactors to absorb the excess heat of reaction

This allows subsequent cracking to proceed to a greater extent and thus converts a low-value component of catalytic cycle oils to a useful product

Isomerization is another reaction type that occurs in hydrocracking and accompanies the cracking reaction.

The oleﬁnic products formed are rapidly hydrogenated, thus maintaining a high concentration of high octane isoparafﬁns and preventing the reverse reaction back to straight-chain molecules.

An interesting point in connection with the hydrocracking of these compounds is the relatively small amounts of propane and lighter materials that are produced as compared with normal cracking processes.

The circulation of large quantities of hydrogen with the feedstock prevents excessive catalyst fouling and permits long runs without catalyst regeneration.

Careful preparation of the feed is also necessary in order to remove catalyst poisons and to give long catalyst life.

Frequently the feedstock is hydrotreated to remove sulfur and nitrogen compounds as well as metals before it is sent to the ﬁrsthydrocracking stage or, sometimes, the ﬁrst reactor in the reactor train can be used for this purpose.

Hydrocracking catalyst is susceptible to poisoning by metallic salts, oxygen, organic nitrogen compounds, and sulfur in the feedstocks.

The feedstock is hydrotreated to saturate the oleﬁns and remove sulfur, nitrogen, and oxygen compounds.

Molecules containing metals are cracked and the metals are retained on the catalyst.

The nitrogen and sulfur compounds are removed by conversion to ammonia and hydrogen sulﬁde.

Although organic nitrogen compounds are thought to act as permanent poisons to the catalyst, the ammonia produced by reaction of the organic nitrogen compounds with hydrogen does not affect the catalyst permanently

This is a beneﬁcial effect when maximizing gasoline production as it conserves hydrogen and produces a higher octane product.

In the hydrotreater a number of hydrogenation reactions, such as oleﬁn saturation and aromatic ring saturation, take place, but cracking is almost insigniﬁcant at the operating conditions used.

In addition to the removal of nitrogen and sulfur compounds and metals, it is also necessary to reduce the water content of the feed streams to less than 25 ppm because, at the temperatures required for hydrocracking, steam causes the crystalline structure of the catalyst to collapse and the dispersed rare-earth atoms to agglomerate.

Water removal is accomplished by passing the feed stream through a silica gel or molecular sieve dryer

On the average, the hydrogen treating process requires approximately 150 to 300 ft3 of hydrogen per barrel of feed (27 to 54 m3 hydrogen per m3 feed).

For most feedstocks the use of a single stage will permit the total conversion of the feed material to gasoline and lighter products by recycling the heavier material back to the reactor.

The process ﬂow for a two-stage reactor is shown in Figure 7.2. If only one stage is used, the process ﬂow is the same as that of the ﬁrst stage of the two-stage plant except the fractionation tower bottoms is recycled to the reactor feed.

The guard reactor usually has a modiﬁedhydrotreating catalyst such as cobalt-molybdenum on silica-alumina to convert organic sulfur and nitrogen compounds to hydrogen sulﬁde, ammonia, and hydrocarbons to protect the precious metals catalyst in the following reactors.

The hydrocracking reactor(s) is operated at a sufﬁciently high temperature to convert 40 to 50 vol% of the reactor efﬂuent to material boiling below 400°F (205°C).

The liquid product from the separator is sent to a distillation column where the C4 and lighter gases are taken off overhead, and the light and heavy naphtha, jet fuel, and diesel fuel boiling range streams are removed as liquid sidestreams.

The fractionator bottoms are used as feed to the second-stage reactor system.

The unit can be operated to produce all gasoline and lighter products or to maximize jet fuel or diesel fuel products.

At normal reactor conditions a 20°F (10°C) increase in temperature almost doubles the reaction rate, but does not affect the conversion level as much because a portion of the reaction involves material that has already been converted to materials boiling below the desired product end point.

As the run progresses it is necessary to raise the average temperature about 0.1 to 0.2°F per day to compensate for the loss in catalyst activity.

Heavy polynuclear aromatics are formed in small amounts from hydrocracking reactions and, when the fractionator bottoms is recycled, can build up to concentrations that cause fouling of heat exchanger surfaces and equipment.

Steps such as reducing feed end point or removal of a drag stream may be necessary to control this problem